Synthesis, Structure, and Transannular π−π Interaction of Three- and Four-Layered [3.3]Paracyclophanes

The synthesis of three- and four-layered [3.3]paracyclophanes ([3.3]PCPs) 3−5 has been accomplished by utilizing the (p-ethylbenzenesulfonyl)methyl isocyanide (EbsMIC) method. The structures of the three- to four-layered [3.3]PCPs 3−5 and their diones 8, 10, and 11 have been elucidated based on the 1H NMR spectra and finally by X-ray structural analysis. In the three-layered [3.3]PCP-dione 8, the trimethylene bridges of the [3.3]PCP unit assume a chair conformation similar to that of 2, while the [3.3]PCP-2,11-dione unit assumes a boat conformation different from that of [3.3]PCP-dione 1 with a chair conformation. On the other hand, the two [3.3]PCP units in three-layered [3.3]PCP 3 both assume a boat conformation. In the four-layered [3.3]PCP-dione 10, the two outer [3.3]PCP units assume a boat conformation while the inner dione unit has a chair conformation. The trimethylene bridges in the four-layered [3.3]PCP 4 are highly disordered even at −150 °C. All the outer benzene rings are distorted into a boat form while the inner ones are distorted into a twist form. In the electronic spectra, bathochromic shift and hyperchromic effect are observed, but the magnitude decreases with an increase in the number of layers and the spectra become structureless. In the charge-transfer (CT) bands of the three- to four-layered [3.3]PCPs 3−5 with tetracyanoethylene (TCNE), two absorption maxima (λmax) are observed. The effect of an increase in the layers becomes significant, and the changes in the longest wavelength λmax values from two to three and three to four are ca. 60 and 50 nm, respectively. By comparison of the stereoisomeric four-layered [3.3]PCPs 4 (meso) and 5 (racemic), the helical arrangement of the trimethylene bridges of 5 shows a more efficient transannular π-electronic interaction. In the three- to four-layered [3.3]PCP-diones, a magnitude of the CT interaction almost comparable to that of [3.3]PCP 2 was observed, and this indicates that the −CH2COCH2− bridges inhibit the CT interaction and that this tendency is supported by the calculated HOMO energy levels and observed oxidation potentials. Three- and four-layered [3.3]PCPs 3−5 show reversible redox processes, and 4 and 5 show an electron-donating ability almost comparable to that of [36]CP. Very good correlation between the λmax of the CT bands with TCNE and the oxidation potentials is observed.

通过采用(对乙苯磺酰)甲基异氰酸酯(EbsMIC)方法，已成功合成了三层和四层[3.3]代亚环状芳烃([3.3]PCPs)3−5。基于1H NMR光谱和最终的X射线结构分析，已阐明三层至四层[3.3]PCPs 3−5及其二酮8、10和11的结构。在三层[3.3]PCP-二酮8中，[3.3]PCP单元的 trimethylene 桥呈现出与2相似的椅式构象，而[3.3]PCP-2,11-二酮单元呈现出与[3.3]PCP-二酮1不同的船式构象，后者具有椅式构象。另一方面，三层[3.3]PCP 3中的两个[3.3]PCP单元都呈现出船式构象。在四层[3.3]PCP-二酮10中，两个外部的[3.3]PCP单元呈现出船式构象，而内部的二酮单元则具有椅式构象。在四层[3.3]PCP 4中，trimethylene 桥在-150°C下仍表现出高度的无序性。所有外部的苯环均扭曲成船形，而内部的则扭曲成扭曲形。在电子光谱中，观察到红移和增色效应，但随着层数的增加，这些效应的幅度减小，光谱变得无序。在三层至四层[3.3]PCPs 3−5与四氰乙烯(TCNE)的电荷转移(CT)带中，观察到两个吸收最大值(λmax)。随着层数的增加，这种效应变得显著，最长波长λmax值从两层增加到三层，以及从三层增加到四层，分别变化约60和50纳米。通过比较立体异构的四层[3.3]PCPs 4(外消旋)和5(对映异构)，5中trimethylene 桥的螺旋排列显示出更有效的跨环π电子相互作用。在三层至四层[3.3]PCP-二酮中，观察到与[3.3]PCP 2几乎相当的CT相互作用强度，这表明-CH2COCH2-桥抑制了CT相互作用，这一趋势得到计算HOMO能级和观测到的氧化势的证实。三层和四层[3.3]PCPs 3−5显示出可逆的氧化还原过程，4和5显示出与[36]CP几乎相当的电供体能力。观察到CT带λmax与氧化势之间具有很好的相关性。